I really don't think the project is in competition with MAG. I've been a Bungie follower forever - pre-Halo - and I really didn't think they were going with a shooter as their first non-Halo project. Really.
Besides, the MAG trailer was cool, certainly, but it wasn't like that they showed ANY reason to believe that game was anywhere near completion or even playable in any form. It was just a CG trailer showing the concept. The guy in the show even stressed that it was a REALLY early look at it.
I can't imagine they'd pull it because it couldn't compete with something that hasn't even made an appearance yet.
Plus, Bungie has always felt that anything beyond 6 v 6 is too large - that players don't have enough of an effect on the outcome to keep things interesting for the player. They have said this repeatedly and forever. It's why Halo never grew betond Big Team 8v8. I can't imagine the company would change its whole philosophy overnight and suddenly want 256-player battles.







